December 5, 2014 at 13:27A bit about Forward Error Correction technology
One of the main limitations in the design of long optical transport networks is the signal-to-noise ratio (OSNR). WDM networks must operate within the acceptable OSNR limits to ensure proper system operation.
The OSNR threshold value is one of the key parameters that determine how far signals can be transmitted without the need for 3R regeneration.
For the formation of data transmission channels with a speed above 10 Gbit, complex optical signal modulation mechanisms are used to achieve a similar transmission range of 1-10 Gbit communication channels. These modulation formats are necessary to minimize the effects of such optical phenomena as chromatic and polarization mode dispersion, as well as to generate an optical signal that complies with ITU 100/50-GHz standards, which is used in modern DWDM systems. The disadvantage of high-speed data transmission channels is the fact that they require a significantly higher OSNR ratio than conventional transmission systems (1-10 Gbps).
On 100 Gbit systems, the minimum OSNR should be 10 dB higher than for signals on 10 Gbit systems. Without any correction or compensation, OSNR limits 100G data transmission to very short distances; at the moment, the maximum transmission range is 40 km over standard single-mode fiber. However, thanks to modern Forward Error Correction (FEC) methods, especially the Soft decision FEC algorithm, it is possible to extend the transmission of high-speed signals over long distances.
Forward Error Correction (FEC) is a signal coding / decoding technique with the ability to detect errors and correct information using the feedforward method. Thus, the receiving equipment can detect and correct errors that occur in the transmission channel. FEC dramatically reduces the number of bit errors (BER), which allows you to increase the transmission distance of the signal without regeneration.
There are several FEC coding algorithms that vary in complexity and performance. One of the most common first-generation FEC codes is the Reed-Solomon code (255, 239). This code adds a little - 7% of the test bytes and about 6 dB of additional OSNR margin, but for high-speed optical networks, a 6 dB increase is an improved performance indicator, increasing the distance between the regenerators by about four times.
In addition to the Reed-Solomon code, some manufacturers offer more complex second-generation FEC coding schemes, for example, the preventive parameter for 10G and 40G optical interfaces. These algorithms, called “ultra” FEC or “enhanced” FEC (EFEC), also use no more than 7% of the transmitted frame, but they contain more complex encoding / decoding algorithms that provide greater OSNR gain - from 2 to 3 dB rather than the Reed-Solomon code.
Along with the development of the first generation - Reed-Solomon FEC and the second generation - EFEC, which allowed to significantly improve the performance for 10G and 40G signals, a more productive third-generation FEC solution was developed, providing increased range and optimal performance for 100G high speed data channels.
The third-generation FEC solution is based on even more powerful encoding / decoding and iterative coding algorithms. In hard decision FEC — the decoding unit determines a “solid” decision based on the incoming signal and initializes one bit of information as “1” or “0” by comparing it with a threshold value. Values above the set threshold are defined as “1”, and values below are defined as “0”. The decoder uses extra bits to provide a more detailed and accurate indication of the incoming signal. In other words, the decoder not only determines based on the threshold value whether the input signal is “1” or “0”, but also provides a reliability factor for “decision making”. The reliability coefficient is determined by an indicator showing how much the signal is above or below the threshold value.
Using the reliability coefficient or “probability” of bits together with more sophisticated third-generation FEC coding algorithms allows the SD-FEC decoder to provide an additional increase in OSNR of 1-2 dB. While an increase in OSNR of 1-2 dB does not sound impressive, it can be interpreted as a possible increase in distance by 20-40%, which is a significant indicator for 100G.
One of the disadvantages of soft decision FEC is the fact that it requires ~ 20% of the transmitted frame, which is more than two times more than the occupied FEC volume of the first and second generation.
With the increase in speed in the data channel from 10G to 100G, the OSNR requirement increased by 10 dB. Without a certain type of compensation or correction, the length of the tracks with a channel speed of 100G will be very limited and uneconomical.
First and second generation FEC algorithms were used on 10G and 40G to reduce BER and increase distance. SD-FEC is the third generation coding algorithm, providing data transmission for 100G optical networks over long distances and with a large relay section.
The OSNR threshold value is one of the key parameters that determine how far signals can be transmitted without the need for 3R regeneration.
For the formation of data transmission channels with a speed above 10 Gbit, complex optical signal modulation mechanisms are used to achieve a similar transmission range of 1-10 Gbit communication channels. These modulation formats are necessary to minimize the effects of such optical phenomena as chromatic and polarization mode dispersion, as well as to generate an optical signal that complies with ITU 100/50-GHz standards, which is used in modern DWDM systems. The disadvantage of high-speed data transmission channels is the fact that they require a significantly higher OSNR ratio than conventional transmission systems (1-10 Gbps).
On 100 Gbit systems, the minimum OSNR should be 10 dB higher than for signals on 10 Gbit systems. Without any correction or compensation, OSNR limits 100G data transmission to very short distances; at the moment, the maximum transmission range is 40 km over standard single-mode fiber. However, thanks to modern Forward Error Correction (FEC) methods, especially the Soft decision FEC algorithm, it is possible to extend the transmission of high-speed signals over long distances.
Forward Error Correction (FEC) is a signal coding / decoding technique with the ability to detect errors and correct information using the feedforward method. Thus, the receiving equipment can detect and correct errors that occur in the transmission channel. FEC dramatically reduces the number of bit errors (BER), which allows you to increase the transmission distance of the signal without regeneration.
There are several FEC coding algorithms that vary in complexity and performance. One of the most common first-generation FEC codes is the Reed-Solomon code (255, 239). This code adds a little - 7% of the test bytes and about 6 dB of additional OSNR margin, but for high-speed optical networks, a 6 dB increase is an improved performance indicator, increasing the distance between the regenerators by about four times.
In addition to the Reed-Solomon code, some manufacturers offer more complex second-generation FEC coding schemes, for example, the preventive parameter for 10G and 40G optical interfaces. These algorithms, called “ultra” FEC or “enhanced” FEC (EFEC), also use no more than 7% of the transmitted frame, but they contain more complex encoding / decoding algorithms that provide greater OSNR gain - from 2 to 3 dB rather than the Reed-Solomon code.
Along with the development of the first generation - Reed-Solomon FEC and the second generation - EFEC, which allowed to significantly improve the performance for 10G and 40G signals, a more productive third-generation FEC solution was developed, providing increased range and optimal performance for 100G high speed data channels.
The third-generation FEC solution is based on even more powerful encoding / decoding and iterative coding algorithms. In hard decision FEC — the decoding unit determines a “solid” decision based on the incoming signal and initializes one bit of information as “1” or “0” by comparing it with a threshold value. Values above the set threshold are defined as “1”, and values below are defined as “0”. The decoder uses extra bits to provide a more detailed and accurate indication of the incoming signal. In other words, the decoder not only determines based on the threshold value whether the input signal is “1” or “0”, but also provides a reliability factor for “decision making”. The reliability coefficient is determined by an indicator showing how much the signal is above or below the threshold value.
Using the reliability coefficient or “probability” of bits together with more sophisticated third-generation FEC coding algorithms allows the SD-FEC decoder to provide an additional increase in OSNR of 1-2 dB. While an increase in OSNR of 1-2 dB does not sound impressive, it can be interpreted as a possible increase in distance by 20-40%, which is a significant indicator for 100G.
One of the disadvantages of soft decision FEC is the fact that it requires ~ 20% of the transmitted frame, which is more than two times more than the occupied FEC volume of the first and second generation.
With the increase in speed in the data channel from 10G to 100G, the OSNR requirement increased by 10 dB. Without a certain type of compensation or correction, the length of the tracks with a channel speed of 100G will be very limited and uneconomical.
First and second generation FEC algorithms were used on 10G and 40G to reduce BER and increase distance. SD-FEC is the third generation coding algorithm, providing data transmission for 100G optical networks over long distances and with a large relay section.